Series capacitor installations are used to increase the transmission capability of power lines for transmission of electric power and for stabilizing the operation of such power lines, especially in the case of long high-voltage transmission lines. For high system voltages, typically exceeding 145 kV, the components included in the installation are usually arranged on a platform insulated towards ground potential.
A known design of such a series capacitor installation and, in particular, protective equipment for such an installation is described in IEEE Transactions on Power Delivery, Vol. 4, No. 2, April 1989, pages 1369-1376: EHV Series Capacitor Banks, a new approach to platform to ground signalling, relay protection and supervision (M Adolfsson et al).
The protective equipment comprises a number of relay protection devices to which measured values of currents sensed in the installation are supplied and which, in dependence on these measured values and on criteria chosen and set in the relay protection devices for evaluation of the measured values, initiate protective measures in the event that these measured values indicate an abnormal operating condition for the installation. The relay protection devices are placed in a control room at ground potential.
The series capacitor is normally designed comprising a number of capacitor units. As is clear from FIG. 3.1 in the cited document, the capacitor units are divided into two parallel-connected capacitor groups, whereby each one of these groups is built up of a plurality of capacitor units.
In addition thereto, the installation comprises a non-linear voltage-limiting resistor, connected in parallel with the series capacitor, in the form of a varistor, a bypass breaker and a spark gap capable of being fired, the last two components being connected in parallel with the series capacitor via a damping inductor D.
A current transformer senses the current that flows through the series capacitor up to the point where this is connected in parallel with the varistor, and a measured value of this current is supplied to a relay protection device of inverse-time type to protect the series capacitor against thermal overload. In this case, the criterion is the current/inverse-time characteristic of the relay protection device.
Another current transformer senses the current which flows through the series capacitor installation out into the power line, that is, the line current, and a measured value of this current is supplied to a relay protection device for detecting subsynchronous oscillations in the current flowing through the power line. Here the criterion is the presence of oscillations in the line current of a frequency considerably lower than the system frequency of the power line.
Another current transformer is connected in a bridge connection between series-connected parts of the two capacitor groups for monitoring their mode of operation. A measured value of current sensed by this current transformer is supplied to a relay protection device for detecting unbalance in the series capacitor, caused, for example, by the triggering of an internal fuse in the capacitor units. In this case, the criterion is a current of a certain level through the current transformer, usually put in relation to the actual line current such that the criterion consists of a quotient between these currents.
The components included in the installation are placed on insulators on the platform, whereby one side of the above-described parallel connection of the components, in FIG. 3.1 in the cited document the right one, is galvanically connected to the platform. For example, a flashover across an insulator causes a current to flow through this connection. A current transformer senses this current and a measured value thereof is supplied to a relay protection device for detecting flashover against the platform. The criterion is here a chosen current level.
A further current transformer senses a current flowing through the spark gap and a measured value thereof is supplied to the relay protection device for detecting any remaining current through the spark gap. In this case, the criterion is a chosen current level that remains for a chosen period of time.
In all of the above-mentioned cases, the respective relay protection device initiates, as protective measure, a closing of the bypass breaker by generating an order signal therefor.
An additional current transformer senses a current flowing through the varistor and a measured value of this current is supplied to a relay protection device for the combined protection of the varistor against overcurrent and against thermal overload. Here the criteria are both a current level and a calculated thermal heating of the varistor. The relay protection device initiates, as protective measure, firing of the spark gap as well as closing of the bypass breaker.
The above-mentioned criteria in some cases also include chosen time delays and the protective measures also include, depending on which relay protection device initiates these, such measures as temporary or definite blocking of the by-pass breaker, automatic reclosing of the installation through reopening of the bypass breaker, etc. Further details about the performance of these protective functions, however, fall outside the scope of this patent application. They are known per se to the person skilled in this technical field and are described in greater detail also in the article referred to here.
In the same article, a so-called optical current transformer suited for the purposes described above, is also described. The current transformer has a magnetic core surrounding a connection bar, which bar is intended to be connected in the conductor whose current is to be measured. Further, the current transformer comprises, in a known manner, a secondary winding with a burden as well as an optoelectric converter for conversion of the voltage, formed across the burden, into an optical signal corresponding to the amplitude of the voltage. The optical signal is transferred to ground potential via light guides connected between the optoelectric converter at the current transformer and an optoelectric converter, arranged at ground potential, which converts the optical signal into a shape and level adapted to the respective relay protection device. Although not shown in FIG. 3.1 in the cited document, the light guide from the respective current transformer is usually passed via a so-called platform link to a common joint box located on the platform, and from this all the light guides are then passed to ground potential via a common insulator in a so-called high-voltage link (FIG. 2.1 in the cited document).
Thus, the protective equipment comprises a large number of measuring points for current-measuring devices, and for each one of these there is required, in addition to the current transformer itself, a platform link for passing the respective light guide to the joint box. Each current-measuring device with its associated platform link entails, in addition to a material cost, also a cost for installation and commissioning. To this is added the fact that each component in an installation in principle increases the risk of a fault arising in the installation.